1. Field of the Invention
The present invention relates to a liquid-crystal display device of active matrix type having an array of pixels and an array of thin film transistors which func tion as switching elements for driving the pixels.
2. Description of the Related Art
Recently, flat display devices have been developed. Due to the use of flat displays, various apparatuses have been made thin. There is now a great demand for liquid-crystal display devices of active matrix type which can display color images and have high pixe density. The recent trend in the art is to use more and more color liquid-crystal display devices in projectors and OA (Office Automation) apparatuses. Hence, it is demanded that liquid-crystal display devices of active matrix type be provided which have various sizes, large and small, and high pixel densities.
A liquid-crystal display device of active matrix type comprises an array of pixels and an array of thin-film transistors (hereinafter referred to as "TFT"), each provided for one pixel. The TFTs function as switching elements. When turned on, they supply video signals to the pixels, thereby driving the pixels in a quasi-static manner. The pixels respond to the video signals at high speed, and no crosstalk involves in supplying the video signals to the pixels. Therefore, the display device of active matrix type can display highresolution, high-contrast images.
With reference to FIG. 1, it will be explained how the liquid-crystal display device of active matrix type perform its function. As is shown in FIG. 1, the device comprises a plurality of parallel scanning lines 11 (i.e., column-selecting lines), a plurality of parallel signal lines 12 (i.e., row-selecting lines) intersecting with the scanning lines 11, a plurality of TFTs 13 (i.e., switching elements) located at the intersections of the lines 11 and 12, a plurality of liquid-crystal layers 14 located at the intersections of the lines 11 and 12, a plurality of pixel capacitors 15 located at the intersections of the lines 11 and 12, a scanning circuit 16 connected to the scanning lines 11, and a signal-holding circuit 17 connected to the signal lines 12. The gate of each TFT 13 is connected to a scanning line 11, whereby the liquid-crystal layer 14 and the pixel capacitor 15 located near the TFT 13 are electrically connected to the signal line 11. The scanning circuit 16 sequentially supplies gate pulses to the scanning lines 11. The signal-holding circuit 17 supplies one-line video signals to the signal lines 12 in synchronism with the gate pulses. Each TFT 13 remains on while a gate pulse is being supplied to the scanning line 11 to which its gate is connected. The pixel capacitor 15 coupled to the TFT 13 accumulates the electrical charge which corresponds to the video signal supplied through the signal line 12 to which the TFT 13 is connected. Hence, the liquid-crystal layer 14 connected to the capacitor 15 is driven. The TFT 13 is turned off the moment a gate pulse is supplied to the next scanning line 11. The pixel capacitor 15 holds the electrical charge until a gate pulse is supplied to the scanning line 11 to which the gate of the TFT 13 is connected. As a result of this, the liquid-crystal layer 14 is continuously driven.
FIGS. 2 and 3 illustrate one of the pixels of the liquid-crystal display device of the active matrix type, which has been described above. More precisely, FIG. 2 is a plan view showing the pixel-array substrate, and, FIG. 3 is a sectional view taken along line III-III in FIG. 2. A TFT 13 is formed on a lower glass substrate 20. It comprises a gate electrode 21, a gate insulating film 22, a semiconductor layer 23, a drain electrode 24, and a source electrode 25--all formed on the lower glass substrate 20. The gate electrode 21 and the drain electrode 24 are integral with a scanning line 11 and a signal line 12, respectively. The TFT 13 further comprises a display electrode 26 and an auxiliary capacitor line 27. The display electrode 26 is connected to the source electrode 25 The auxiliary capacitor line 27 is formed between the lower substrate 20 and the gate-insulating film 22 and located below the display electrode 26. Hence, an auxiliary capacitor is formed between the line 27 and that part of the electrode 26 which is located right above the line 27. The auxiliary capacitor, thus formed, increases the capacitance of the pixel capacitor 15, compensates for the leakage current flowing through the TFT 13 while the capacitor 15 holds the video signal, and moderates the changes in the potential of the display electrode 26 which occur due to the capacitive coupling between the electrode 26 and the other electrodes.
The pixel further comprises a common eectrode 29 formed on that surface of an upper glass substrate 28 which opposes the lower glass substrate 20. A liquid-crystal layer 14 is interposed between the lower and upper glass substrates 20 and 28. An electric field is generated between the display electrode 26 and the common electrode 29, to drive that portion of the liquid-crystal layer 14 which contacts the display electrode 26.
The pixels of the liquid-crystal display device of active matrix type are formed by forming layers and films and by performing photolithography for microprocessing these layers and films. It is difficult to manufacture a large-screen device having no defects. Since the device has a complex electrode arrangement and a multi-layered structure, short-circuiting is likely to occur between the electrodes. In most cases, short-circuiting takes place between the gate electrode 21 and the source electrode 24 which are spaced apart by the gate-insulating film 22, and between the display electrode 26 and the auxiliary capacitor line 27 which are spaced apart by the film 22. When this short-circuiting occurs, the potential of the display electrode 26 can no longer have a predetermined value. In this case, the pixel fails to operate well, inevitably degrading the quality of the image being displayed on the screen of the liquid-crystal display device. In particular, when the pixel forms a bright spot on the screen, it lowers the quality of the image. The short-circuiting between the electrodes is one of the causes which decrease the yield of the liquid-crystal display device of active matrix type.
If such a defective spot is found in the image displayed, the pixel forming this defective spot must be repaired. In other words, the short-circuited portion of the pixel must be removed. Various methods of removing the short-circuited portion have been proposed. Among these is the method in which a laser is employed. If the display electrode 26 and the auxiliary capacitor line 27, for example, are short-circuited to each other, a laser beam is applied to those parts Da of the line 27, thus cutting off these parts Da as is illustrated in FIG. 2.
This method of removing the short-circuited portion of a pixel results in the elimination of the auxiliary capacitance, that is, a decrease in the capacitance of the pixel capacitor 15. Consequently, the liquid-crystal pixel may fail to operate well, due to the leakage current of the TFT 13 or the capacitive coupling between the display electrode 26 and any other electrode. More specifically, the potential of the electrode 26 inevitably deviates from the predetermined value, and the defective spot cannot be rendered completely imperceptible. Further, the technique of removing the short-circuited portion of the pixel cannot be applied to the case where an electrical charge is applied to the auxiliary capacitance line 27 from only end thereof, or the case where the gate and source electrodes 21 and 25 are short-circuited to each other.